Gas turbine inlet silencer

ABSTRACT

A system for attenuating sound emissions from an inlet to a flow path for an air inducting machine including an inlet duct structure having inlet and outlet passages. A planar vortex generator is located adjacent to the outlet passage and creates vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave at the vortex generator. The standing wave has an upstream propagating component that reflects off an acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.

FIELD OF THE INVENTION

The present invention relates to gas turbine engines and, more particularly, to an inlet silencer for attenuating sound emissions from an inlet to the gas turbine engine.

BACKGROUND OF THE INVENTION

Gas turbine engines used for power generation include a compressor for supplying the engine with compressed air that is mixed with fuel and ignited to produce a hot working gas, and the hot working gas is directed through a turbine section to produce a work output from the engine. The compressor includes a plurality of stages formed by pairs of rows rotating blades and stationary vanes. The rotating blades produce an inlet noise that results from interaction of various different phenomena related to interaction of the blades with air being drawn into the compressor, and produces both broadband noise and blade-passing discrete tones.

Various mechanisms have been implemented to suppress the noise emitted from the compressor inlet. For example, acoustically absorptive parallel inlet baffles may be installed at the inlet duct for the compressor, between an inlet filter house and the compressor, to absorb compressor inlet noise. However, such baffle structure may restrict air flow and substantially increase the cost for providing noise attenuation at the compressor inlet.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention a system is provided for attenuating sound emissions from an inlet to a flow path for an air inducting machine. The system comprises an inlet duct structure having an inlet passage and an outlet passage downstream from the inlet passage, the outlet passage defining an outlet plane extending span-wise generally perpendicular to flow through the outlet passage. A vortex generator is located adjacent to or upstream of the outlet passage, the vortex generator extending across the outlet passage and defining a plane. The vortex generator creates vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave. An acoustic reflector wall is located on the inlet duct between the inlet and outlet passages, upstream of the vortex generator and oriented generally parallel to the plane of the vortex generator. The standing wave has an upstream propagating component that reflects off the acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.

The vortex generator may include a plurality of vortex producing rods extending in a row parallel and in spaced relation to each other in the span-wise direction across the outlet passage, and the rods form wake shed vortices on downstream sides thereof in a plane generally parallel to the outlet plane.

The rods may have a circular cross-section defining a diameter, and at least one of the rods may have a different diameter than at least another of the rods.

The diameters of particular rods may be selected with reference to an average velocity of air flow at the location of each of the particular rods.

A distance from the rods to the acoustic reflector wall may vary, depending on the diameter of the rod.

Two or more rows of the rods may be spaced from each other in the direction of flow through the outlet passage wherein rods in the first row of rods may be aligned with rods in the second row of rods in a direction extending perpendicular to the outlet plane, and may form either an in-line or staggered array.

The acoustic reflector wall may be located at a junction where the flow direction changes between the inlet and outlet passages, parallel to the plane of the vortex generator.

The air inducting machine may be a gas turbine engine having a compressor including rotating blade rows, and the specific tonal acoustic frequency may be a blade passing frequency created by at least one of the rotating blade rows.

The standing wave may have a frequency that corresponds to the blade passing frequency, and the vortex generator may be positioned relative to the acoustic reflector wall such that a distance, d, between the standing wave and the acoustic reflector wall is equal to n(λ/4), where n is an odd integer and λ is the wavelength of the blade passing frequency.

At least one of the vortex generator and the acoustic reflector wall may be movable relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance, d.

Interior surfaces of the inlet duct structure, except for the acoustic reflector wall, may be lined with an acoustic absorptive structure.

In accordance with another aspect of the invention, a method of attenuating sound emissions from an inlet to a flow path for an air inducting machine is provided. The method comprises providing a flow of air through an inlet duct structure having an inlet passage and an outlet passage, the outlet passage defining an outlet plane extending span-wise generally perpendicular to flow through the outlet passage; directing the flow of air over a vortex generator located adjacent to or upstream of the outlet passage to create vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave; providing an acoustic reflector wall located on the inlet duct between the inlet and outlet passages upstream of the vortex generator and oriented generally parallel to the plane of the vortex generator; and wherein the standing wave has an upstream propagating component that reflects off the acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.

The step of directing the flow of air over a vortex generator may comprise providing a plurality of rods extending in a row parallel and in spaced relation to each other, and directing the flow of air through spaces between the rods. The standing wave may be formed in a plane spaced downstream from the row of rods, and a plurality of the rods may be located at different distances from the plane of the standing wave.

The method may further include forming the standing wave such that it has a frequency that destructively interferes with the specific tonal acoustic frequency after the upstream propagating component reflects off the acoustic reflector wall.

The method may further include moving at least one of the vortex generator and the acoustic reflector wall relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance between the vortex generator and the acoustic reflector wall to tune the reflected component so as to destructively interfere with the specific tonal acoustic frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein:

FIG. 1 is a diagrammatic view of a gas turbine engine configured to incorporate aspects of the invention;

FIG. 2 is a cross-sectional view of an inlet duct including a sound attenuation system in accordance with aspects of the invention;

FIG. 3 is a cross-sectional view of the inlet duct taken along line 3-3 in FIG. 2;

FIG. 4 is an enlarged view of area A in FIG. 2, diagrammatically illustrating a standing wave;

FIG. 5 is a cross-sectional view taken along line 5-5 in FIG. 3;

FIG. 6 is a cross-sectional view illustrating a vortex generator having different size rods; and

FIGS. 7A and 7B are cross-sectional views showing alternative embodiments of vortex generators having two rows of rods.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.

Referring to FIG. 1, in accordance with an aspect of the invention, an inlet silencer is provided at the inlet to a flow path for an air inducting machine. In the illustrated embodiment, the air inducting machine is a gas turbine engine 10, such as a gas turbine engine 10 used in a power generation plant. The engine 10 includes a compressor 12 that receives air through a compressor inlet duct structure or inlet duct 14. The illustrated inlet duct may be located downstream from an inlet filter house (not shown). The compressor delivers compressed air to a combustor 16 where the air is combusted with a fuel to produce a hot working gas for a turbine section 18 of the engine 10, producing a work output such as to power an electric generator (not shown).

Referring to FIGS. 2 and 3, the inlet duct 14 of the illustrated embodiment has a generally rectangular cross-section, however, it may be understood that the aspects of the invention described herein may be implemented in inlet ducts 14 having other configurations, such as a circular configuration. The inlet duct 14 includes an inlet passage 28 formed by first and second side walls 20, 22, and outer and inner walls 24, 26 which connect the side walls 20, 22. As seen in FIG. 2, the inlet passage defines a first central axis A₁.

An outlet passage 30 is located between the inlet passage 28 and a compressor inlet housing 32, and an expansion joint 34 may be provided at the connection between the outlet passage 30 and the compressor inlet housing 32. The outlet passage 30 includes first and second side walls 36, 38, which may be contiguous with the first and second side walls 20, 22 of the inlet passage 28. The first and second walls 36, 38 are connected by a front wall 40 that extends to the inner wall 26 of the inlet passage 28, and a rear wall 42 that extends to the outer wall 24 of the inlet passage 28. The outlet passage 30 defines a second central axis A₂ that may be generally perpendicular to the first central axis A₁ of the inlet passage 28. An air flow F₁ entering the compressor 12 may pass into the inlet duct 14 through the inlet passage 28 generally parallel to the first central axis A₁, and change directions at a junction between the inlet and outlet passages 28, 30, i.e., at an area generally indicated by 44, and further may pass as an air flow F₂ generally parallel to the second central axis A₂ to the compressor inlet housing 32.

It should be noted that although the described inlet duct 14 is depicted as having differently oriented inlet and outlet passages 28, 30 as a preferred embodiment, the inlet and outlet passages 28, 30 may be at an angle other than perpendicular relative to each other, provided only that the plane of the acoustic reflector wall be parallel to the plane of the vortex generator Po.

Referring to FIG. 3, the compressor 12 includes a plurality of rotatable blades 46 arranged as a row of circumferentially distributed blades 46, a first row of which is depicted in FIG. 3. While operation of the turbine engine 10, and in particular the compressor 12, produces a broad range of sound frequencies that are emitted to the inlet duct 14, the rotation of the blades 46 produces specific tonal acoustic frequencies. In particular, rotation of the first row of blades 46 produces a specific tonal acoustic frequency, and at a sufficiently large amplitude, which forms a dominant frequency that aspects of the present invention are configured to attenuate. The dominant or specific tonal acoustic frequency is considered a “pure tone”, which is a signal with a line spectrum consisting of a single frequency that corresponds to a blade passing frequency of the first row of blades 46, and is referred to herein as the “dominant compressor tone”.

In accordance with an aspect of the invention, a vortex generator 48 is provided located adjacent to, upstream from, or in the outlet passage 30. In the embodiment illustrated in FIGS. 2 and 3, the vortex generator 48 is located within an entry to the outlet passage 30 and includes a plurality of cylindrical pipes or rods 50. The rods 50 are arranged in spaced relation to each other in an array or row defining a plane, P_(V), extending across a width, w, of the outlet passage 30. The plane, P_(V), of the array of rods 50 is oriented generally perpendicular to the second central axis A₂ of the outlet passage 30.

Referring to FIG. 4, as will be described in greater detail below, the rods 50 have a diameter, D, selected to produce a wake shed vortex 52 downstream from each rod 50 at a particular frequency, or within a limited frequency range, that overlaps with the dominant compressor tone. The illustrated wake shed vortex 52 downstream from each of the rods 50 is commonly known as a von Karman vortex street. In addition to being provided with a particular diameter, D, the rods 50 are separated by a center-to-center spacing, X (see FIG. 5), to permit sufficient flow past the sides of the rods 50 to form a von Karman vortex street, as well as being located sufficiently close to enable the wake shed vortex 52 of each rod 50 to interact with the wake shed vortices 52 of adjacent rods 50.

Since the engine 10 is used in a power generating plant, which operates at a design speed for any load, the rotational speed of the blades 46 will remain substantially constant, such that the dominant compressor tone will not vary substantially throughout the operation of the engine 10. Further, since gas turbine engines are “constant volume machines,” the velocity of the air flow drawn into the inlet duct 14 and past the rods 50 will remain substantially constant. Hence, the wake shed vortices 52 formed downstream of the rods 50 will be in a substantially constant frequency relationship with respect to the dominant compressor tone.

It may be noted that the frequency of the wake shed vortices 52 will normally wander or vary within a range of frequencies, such that the frequency along the span of a rod 50 is not typically constant. However, by superimposing an intense sound field, such as is provided by the dominant compressor tone, E1, on the nominal shedding frequencies of the rods 50, the shedding frequencies will become in-phase and uniform, i.e., coupled, in a two-dimensional sheet or plane.

In particular, the wake shed vortices 52 constructively interact with the dominant compressor tone emitted from the compressor 12 to create a “lock in” phenomenon which forms a plane of coherent waves at the same frequency as the dominant compressor tone, and depicted as a standing wave 54 in a generally planar region 56 downstream from the rods 50, as illustrated in FIG. 4. The region 56 of the standing wave 54 is typically located one to five rod diameters downstream from the rods 50. The standing wave 54 is perpendicular to the plane, P_(V), of the array of rods. The width, w, may be selected such that a transverse acoustic mode of the rectangular volume defined in the outlet passage 30 is an integer multiple of the wavelength of the standing wave 54, even though the transverse mode is only weakly interacting with the intense dominant compressor tone sound field, E1, and the potential strength of the transverse mode is minimized as a result of an acoustically absorbent wall lining on walls 40 and 42, as is described further below.

The standing wave 54 has an upstream propagating component 58 in the form of a planar wave front that travels toward the junction 44 with the inlet passage 28. An acoustic reflector wall 60 is located supported at the outer wall 24 adjacent to the junction 44 and axially aligned with the array of rods 50. As seen in FIG. 2, the second central axis A₂ passes through both the plane, P_(V), of the rods 50 and the reflector wall 60. The reflector wall 60 is formed of a material, such as a hard wall material, to efficiently acoustically reflect the upstream propagating component 58 of the standing wave 54. The reflector wall 60 is oriented parallel to a plane, P₀ (see, FIG. 4), defined by the planar region 56, such that a reflected wave front 62 is reflected from the reflector wall 60 parallel, and in the opposite direction, to the upstream propagating component 58. The plane, P₀, defines an effective origin of the upstream propagating wave 58.

A distance, d, from the plane, P₀, of the standing wave 54 to the reflector wall 60 is preferably equal to a value of n(λ/4), where n is an odd integer and λ is the wavelength of the standing wave 54. Hence, the reflected wave front 62 has the same wavelength but is one-half wavelength out of phase with the wave front of the upstream propagating component 58 produced at the planar region 56, resulting in destructive interference occurring between the reflected wave front 62 and the upstream propagating component 58, with resulting attenuation of the blade passing frequency.

It may be noted that the destructive interference provided by the sound attenuation system described herein is significant in that the dominant compressor tone within the inlet duct 14 is not typically in the form of a uniform wave front, and therefore would not normally be susceptible to destructive interference. The frequency of the wake shed vortices and the dominant compressor tone cooperate to form the “locked in” wave front that is conducive to reflecting off the reflector wall 60 as a reflected wave front 62 out of phase with the upstream propagating wave front 58 for destructive interference.

Other noise frequencies, such as broad band inlet noise having an amplitude or energy that is lower than the frequency of the dominant compressor tone, may be damped out or attenuated by conventional inlet silencer structures. For example, the side walls 20, 22, 36, 38, a portion of the outer wall 24 not including the reflector wall 60, the inner wall 26, and the front and rear walls 40, 42 may be provided with an acoustically absorptive liner system, such as may be formed by a perforated plate 61 located in front of an acoustically absorptive fiberglass pillow structure 63 (FIG. 2).

The reflector wall 60 may be supported for movement toward and away from the plane, P₀, of the standing wave 54 in order to “tune” the acoustic attenuation system by adjusting the distance, d, to be equal to n(λ/4), where n is an odd integer. For example, the reflector wall 60 may be movable to adjust the distance, d, about 7 to 8 centimeters in a direction along the second central axis A₂. Such tuning of the system may be necessary, for example, to adjust for variations resulting from changes in the ambient temperature of the air passing into the inlet structure 14, and to fine tune the system during installation. As is depicted diagrammatically in FIG. 2, the reflector wall 60 may be actuated for movement relative to the outer wall 24 by actuators 64. The actuators 64 may be any conventional actuator such as, for example, a manually adjustable mechanism, or a linear actuator actuated by a servomotor, pneumatics or hydraulics. Additionally, or instead of, moving the reflector wall 60 to tune the acoustic attenuation system, the array of rods 50 may be actuated for movement relative to the reflector wall 60, such as by the actuators 66 diagrammatically depicted in FIG. 3.

It may be noted that the frequency of the wake shed vortices 52 formed downstream from the rods 50 does not necessarily have to be exactly the same as the dominant compressor tone for the “lock in” phenomena to occur. As long as the acoustic field associated with the dominant compressor tone is within about ±10% to ±20% of the wake shed vortex frequency that would be formed in the absence of the influence of the dominant compressor tone, then the two acoustic fields will “lock in” to form the standing wave 54.

The frequency, f, that would be formed behind the rods 50 in the absence of the acoustic field of the dominant compressor tone is described by the equation relating the vortex frequency in a von Karman vortex street to the Strouhal number as follows:

f=(S·u)/D

where:

-   -   S is the Strouhal number, a dimensionless number that is usually         equal to about 0.2 for an isolated rod;     -   u is the average velocity of flow past the rod (meters/sec); and     -   D is the diameter of the rod, (meters)

From the above equation it can be seen that, in order to match the frequency produced by the von Karman vortex street to the frequency of the dominant compressor tone, the rod diameter, D, may be selected with reference to the velocity of air flowing past the rods 50, which velocity will be substantially determined by the design volume flow for the engine 10.

The velocity profile across the inlet duct, normal to the flow direction, may vary substantially and thus may produce an associated variation in the wake shed frequency across the array of rods 52. For example, the air flow may have a higher velocity toward the center of the outlet passage 30 than adjacent to the walls. In order to maintain a substantially constant frequency across the array of rods 50, a larger diameter of rods 50 may be provided in locations of lower flow velocity, as compared to smaller diameter rods 50 in locations of higher flow velocity. Additionally, since the distance from any given rod 50 to the effective location of the associated vortice's acoustical sound field is a function of the diameter of the rod 50, the distance of each rod 50 to the reflector 60 should be adjusted to place the acoustical sound field of the rods 50 at the location of the plane P₀.

An exemplary array of rods 50 having different diameters is illustrated in FIG. 6, in which it is assumed that a lower flow velocity may be present adjacent to the walls 40, 42 than toward the center of the passage 30. In this illustrated embodiment the smaller diameter rods 50 are located adjacent to the walls 40, 42, and these smaller diameter rods 50 are located closer to the reflector wall 60, such that the array of rods 50 is oriented angled outwardly (i.e., angled upstream) toward the center of the passage 30, as depicted by angles α, β. It should be noted that the variation in diameter of the rods 50 illustrated in FIG. 6 is exaggerated, and the angled array of the rods 50 is depicted with exaggerated angles whereas in that the actual displacement of the rods 50 relative to each other is on the order of a few millimeters, such as a displacement along the second central axis A₂ of about one to ten millimeters.

It should also be noted that, since the velocity field within the inlet duct 14 may also vary along the length of the rods 50, the diameter of one or more of the rods 50 may vary along the rod length. Hence, any rod 50 having a varying diameter may also be effectively angled along the length of the rod 50 to place the acoustical sound field of the rod 50 at the location of the plane P₀.

The frequency of the dominant compressor tone is typically on the order of 1000 Hz and local velocities within the inlet duct 14 may be as low as about 20 m/s. Accordingly, with a Strouhal number of about 0.2, the required rod diameter would be about 4 mm. In accordance with a further aspect, and referring to FIGS. 7A and 7B, an alternative configuration for the acoustic attenuation system is illustrated, including two rows of rods 50 a and 50 b located spaced along the second central axis A₂. As is known in the art, the additional set of rods increases the Strouhal number such that a larger diameter, structurally stronger, rod may be implemented in the vortex generator. FIG. 7A illustrates that the rows of rods 50 a, 50 b may be axially aligned with each other. FIG. 7B illustrates that the rows of rods 50 a, 50 b may be axially displaced relative to each other. Further, it should be noted that within the scope of the invention, more than two rows of rods 50 may be provided.

Additionally, it should be understood that alternative configurations of the acoustic attenuation system relative to the inlet duct 14 may be provided. For example, it is not necessary that the reflector wall 60 be positioned where every change in flow direction occurs in the inlet duct 14, and the reflector wall 60 may be positioned at any upstream location where it can be located parallel to the plane, P₀, of the standing wave 54.

While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention. 

What is claimed is:
 1. A system for attenuating sound emissions from an inlet to a flow path for an air inducting machine, the system comprising: an inlet duct structure having an inlet passage and an outlet passage downstream from the inlet passage, the outlet passage defining an outlet plane extending span-wise generally perpendicular to flow through the outlet passage; a vortex generator located adjacent to or upstream of the outlet passage, the vortex generator extending across the outlet passage and defining a plane, the vortex generator creating vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave; an acoustic reflector wall located on the inlet duct between the inlet and outlet passages, upstream of the vortex generator and oriented generally parallel to the plane of the vortex generator; and wherein the standing wave has an upstream propagating component that reflects off the acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.
 2. The system of claim 1, wherein the vortex generator includes a plurality of vortex producing rods extending in a row parallel and in spaced relation to each other in the span-wise direction across the outlet passage, and the rods form wake shed vortices on downstream sides thereof in a plane generally parallel to the outlet plane.
 3. The system of claim 2, wherein the rods have a circular cross-section defining a diameter, and at least one of the rods has a different diameter than at least another of the rods.
 4. The system of claim 3, wherein the diameters of particular rods are selected with reference to an average velocity of air flow at the location of each of the particular rods.
 5. The system of claim 4, wherein a distance from the rods to the acoustic reflector wall varies, depending on the diameter of the rod.
 6. The system of claim 2, including two or more rows of rods spaced from each other in the direction of flow through the outlet passage wherein rods in the first row of rods are aligned with rods in the second row of rods in a direction extending perpendicular to the outlet plane, and form either an in-line or staggered array.
 7. The system of claim 1, wherein the acoustic reflector wall is located at a junction where the flow direction changes between the inlet and outlet passages, parallel to the plane of the vortex generator.
 8. The system of claim 1, wherein the air inducting machine is a gas turbine engine having a compressor including rotating blade rows, and the specific tonal acoustic frequency is a blade passing frequency created by at least one of the rotating blade rows.
 9. The system of claim 8, wherein the standing wave has a frequency that corresponds to the blade passing frequency, and the vortex generator is positioned relative to the acoustic reflector wall such that a distance, d, between the standing wave and the acoustic reflector wall is equal to n(λ/4), where n is an odd integer and λ is the wavelength of the blade passing frequency.
 10. The system of claim 9, wherein at least one of the vortex generator and the acoustic reflector wall is movable relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance, d.
 11. The system of claim 1, wherein interior surfaces of the inlet duct structure, except for the acoustic reflector wall, are lined with an acoustic absorptive structure.
 12. A method of attenuating sound emissions from an inlet to a flow path for an air inducting machine, the method comprising: providing a flow of air through an inlet duct structure having an inlet passage and an outlet passage, the outlet passage defining an outlet plane extending span-wise generally perpendicular to flow through the outlet passage; directing the flow of air over a vortex generator located adjacent to or upstream of the outlet passage to create vortices that interact with a specific tonal acoustic frequency emitted from the inlet of the machine to effect formation of a standing wave; providing an acoustic reflector wall located on the inlet duct between the inlet and outlet passages upstream of the vortex generator and oriented generally parallel to the plane of the vortex generator; and wherein the standing wave has an upstream propagating component that reflects off the acoustic reflector wall to form a reflected component that interferes with the upstream propagating component to attenuate the specific tonal acoustic frequency from the inlet of the machine.
 13. The method of claim 12, wherein directing the flow of air over a vortex generator comprises providing a plurality of rods extending in a row parallel and in spaced relation to each other, and directing the flow of air through spaces between the rods.
 14. The method of claim 13, wherein the standing wave is formed in a plane spaced downstream from the row of rods.
 15. The method of claim 14, wherein a plurality of the rods are located at different distances from the plane of the standing wave.
 16. The method of claim 12, wherein the standing wave has a frequency that destructively interferes with the specific tonal acoustic frequency after the upstream propagating component reflects off the acoustic reflector wall.
 17. The method of claim 16, including moving at least one of the vortex generator and the acoustic reflector wall relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance between the vortex generator and the acoustic reflector wall to tune the reflected component so as to destructively interfere with the specific tonal acoustic frequency.
 18. The method of claim 12, wherein the air inducting machine is a gas turbine engine having a compressor including rotating blade rows, and the specific tonal acoustic frequency is a blade passing frequency created by at least one of the rotating blade rows.
 19. The method of claim 18, wherein the standing wave has a frequency that corresponds to the blade passing frequency, and the vortex generator is positioned relative to the acoustic reflector wall such that a distance, d, between the standing wave and the acoustic reflector wall is equal to n(λ/4), where n is an odd integer and λ is the wavelength of the blade passing frequency.
 20. The method of claim 19, including moving at least one of the vortex generator and the acoustic reflector wall relative to the other of the vortex generator and the acoustic reflector wall to adjust the distance, d. 